It seems that some other process has /proc/acpi/event file opened. Can you provide output of the following command:
sudo fuser -v /proc/acpi/event
** Changed in: dbus (Ubuntu)
Sourcepackagename: dbus => acpid
Status: Unconfirmed => Needs Info
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[Edgy] issues with dbus, powersaved, acpid and kpowersave
https://launchpad.net/bugs/64646
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